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The serene waters of sky pond, one of the most popular tourist attractions in northeastern Asia, belie the fact that it is nestled inside the crater of one of the region’s most dangerous volcanoes—a peak known as Changbai Mountain to the Chinese and Mount Paektu to Koreans. That 2,744-meter-tall volcano, which straddles the border between China and North Korea, last erupted in 1903 but has displayed signs of awakening in recent years.

The lake is the source of three Asian rivers that, during an eruption, could serve as conduits for lahars—devastating blends of hot ash, mud and water that have the consistency of wet cement. A major eruption could send such flows racing down the volcano’s slopes, threatening hundreds of thousands of people.